It is often of interest to characterize a nonlinear device, such as a diode, transistor, or nonlinear circuit, by stimulating it using one or two pure, large-amplitude sinusoidal signals. In the former, the device responds with a harmonic series. In the later, the device converts the input signal into a spectrum containing energy at intermodulation frequencies. Some exemplary nonlinear quantities that are measured using such one and two-tone stimuli are harmonic distortion, third-order intercept (TOI), and third-order inter-modulation (IM3).
Because superposition is generally not valid for nonlinear devices, the characterization of nonlinear devices depends critically on the nature of the input signals used for their characterization. However, the spectral content of an input signal is often imperfect. That is, when a signal source is asked to provide an ideal sinusoid at a given frequency, f, it actually provides a signal that contains unwanted spectral components at small (complex) amplitudes, at frequencies corresponding to k*f (where k is a positive integer). The unwanted spectral components are often created by the source's output amplifier(s) or mixer(s).
When a device is stimulated with an imperfect input signal, its output is a combination of i) a response to a desired input signal, and ii) a response to the unwanted frequency components that contaminate the desired input signal. Thus, without a way to calibrate the source that provides the input signal, or without a way to correct the measured response of the device, it is impossible to know whether the device's response is the result of i) an intrinsic property of the device, or ii) an imperfect stimulus.
Most current solutions for calibrating nonlinear instruments, such as Nonlinear Vector Network Analyzers (NVNAs) and Large-Signal Network Analyzers (LSNAs), involve crude corrections for gain compression, applied only at the fundamental frequency. There is no rigorous and time-effective procedure for calibrating a source or receiver for unwanted signal components (such as energy at harmonics or intermodulation frequencies of a desired input signal, or reflections due to imperfect load matches at the ports of a device). Even active device measurement applications, such as intermodulation applications (e.g., third-order intercept (IP3) or intermodulation distortion (IMD) applications) are not calibrated for imperfect stimuli, and therefore fail to correct for mismatch at the fundamental or intermodulation frequencies.
Given the scarcity of solutions for calibrating nonlinear instruments for imperfect stimuli, it is often necessary to characterize nonlinear devices using expensive sources that produce signals which are as close to ideal as possible.